Inspection system for heat exchanger tubes

ABSTRACT

A system for inspecting the relatively long, thin-walled, small-bore tubes of a heat exchanger provided with support plates for holding the tubes in predetermined positions, comprising a composite scanner having a plurality of flexure springs contacting the interior surface of a tube for detecting and profiling tube abnormalities as the scanner traverses the tube and an eddy current sensor incorporated in the scanner for generating a unique signal as the scanner passes a contiguous support plate to assist in the accurate location of tube abnormalities.

This invention relates to an inspection system for determining, in-situ,the integrity of heat exchanger tubes. More particularly this inventionrelates to an inspection system for determining, in-situ, in heatexchanger tubes, the location, physical dimensions and character ofabnormalities such as, but not limited to, dents, out-of-roundness,inside diameter variations and the like.

Of critical importance is the identification and measurement of suchabnormalities in steam generators used in nuclear power producing units.Such generators may include upwards of sixteen thousand thin walled,small bore tubes. As an order of magnitude, such tubes may have an O.D.of 0.625", a wall thickness of 0.034" and a length of 60' or more. Thetubes are held in desired conifguration within the generator by aplurality of support plates, distributed along their lengths and byrelatively thick tube sheets at their ends, which also seal the interiorfrom the exterior of the tubes.

Before being placed in service, and during operation it is essentialthat the tubes in such generators be free of significant abnormalities.It is therefore an established requirement that the tubes be inspectedprior to being placed in service and periodically thereafter so that thelocation and physical dimensions of such abnormalities, if any, can bedetermined and a decision made as to the seriousness thereof and thecorrective action to be taken.

The generation of eddy current signatures to identify certain types oftube abnormalities is well established in the art. Reference may bemade, for example, to U.S. Pat. No. 3,302,105 which illustrates anddescribes the eddy current signatures of various types of tubeabnormalities which may be detected by this method. This method is notcapable, however, of accurately measuring as contrasted to detecting,the physical dimensions of tube abnormalities such as dents, and insidediameter variations.

It is therefore an object of this invention to provide an inspectionsystem which will, in-situ, accurately locate and measure the size oftube abnormalities.

A further object of this invention is to provide a system fordetermining the inside diameter of a heat exchanger tube throughout itsentire length.

Still another object of this invention is to provide a system fordetermining the out-of-roundness of a heat exchanger tube throughout itsentire length.

A further object of this invention is to provide a tube inspectionsystem which is simple to operate and requires a minimum of down time ofthe heat exchanger.

Another object of this invention is to provide a system whereby theinspection of heat exchanger tubes located in a hostile environment canbe remotely controlled by an operator located in a benign environment.

These and other objects will be apparent from the following descriptionwhen considered in connection with the drawings in which:

IN THE DRAWINGS

FIG. 1 is a schematic illustration of a composite scanner as applied tothe in-situ inspection of a typical nuclear steam generator tube.

FIG. 2 is a longitudinal cross-section view of the composite scannershown in FIG. 1.

FIG. 2A is a fragmentary illustrating a modification of the scannershown in FIG. 2.

FIG. 3 is an end view of the composite scanner taken along the line 3--3of FIG. 2 in the direction of the arrows.

FIG. 4 is a fragmentary view of a spring finger.

FIG. 5 is an elementary wiring diagram of the measuring circuitsincorporated in the inspection system.

FIG. 6 is a block diagram of the read-out equipment incorporated in theinspection system.

FIG. 7 is a fragmentary view of part of a typical recording of theread-out equipment illustrated in FIG. 6.

DETAILED DESCRIPTION

Referring to the drawings, wherein like reference characters designatelike or corresponding parts throughout the several views, there is shownin FIG. 1 a cross-section of a fragment of a tube 1, as incorporated ina nuclear steam generator, supported at its upper end by a tube sheet 2and at its lower end by a tube sheet 3. The tube sheets 2 and 3 arewelded to the tube 1 so that a fluid circulated through the tube, whichmay be the primary coolant, is isolated from the water and steamsurrounding the exterior of the tube.

Distributed along the length of the tube are a plurality of supportplates 4 holding the tube in desired position. The tube is not securedto the support plates by welding or the like, but passes through, withclose tolerance, holes drilled or otherwise formed in the plates, whichare also provided with passageways for the flow of water and/or steamalong the exterior of the tube. As a tube sheet may, for purposes ofthis invention, be considered a special type of support plate, thegeneric term "support plate" will hereinafter be used.

Shown within the tube 1 is a composite scanner 6 attached to a cable 8for drawing the scanner through the tube at a selected speed, usually inthe order of one foot per second. Various arrangement are known fordrawing a scanner through a tube, one such arrangement, particularlyadapted to the scanning of tubes in a nuclear steam generator, isillustrated and described in U.S. Pat. No. 4,172,492. In making a scan,the scanner is ordinarily positioned to one end of the tube, or to apredetermined bench mark, as, for example, a selected support plate, andthen drawn through the tube by a remotely located drive mechanism (notshown) at the selected speed.

Referring to FIGS. 2 and 3 there is shown, respectively, the scanner 6in longitudinal cross-section and as viewed along the line 3--3 of FIG.2 in the direction of the arrows. A cylindrical body 10 is provided witha plurality, in the embodiment shown eight, cantilever, spring fingers12, equally spaced about the circumference of the body. The springfingers may be formed integral with the body or securely anchoredthereto by any suitable means. Near the free end a section of eachfinger 12 is bent to form an outwardly extending crook 13, necked downto form a cusp 14, as shown in FIG. 4. In making a tube scan, the cusps14 bear against the tube wall and follow changes in contour as may becaused by dents, diameter variations and the like. Preferably, but notessentially, the fingers 12 are enclosed in a protective sheath 16,secured to the body 10, and provided with openings 17 through which thecrooks 13 project.

To each finger 12 there is secured, preferably at or near the point ofmaximum strain adjacent the body 10, a bi-axial strain gauge 18,provided with a winding 19 running parallel with the longitudinal centerline of the finger and a winding 20 running at right angles to thecenter line. The resistance of winding 19 changes in proportion tochanges in ambient temperature and the deflection of the spring fingerfrom a null or neutral position. The resistance of winding 20 issubstantially unaffected by deflection of the spring finger, but changesresistance in proportion to changes in ambient temperature. Each straingauge winding is provided with leads, such as shown at 21, runningthrough passageways formed in the base 10 and through the cable 8 tosignal receiving instrumentation, to be described later.

Incorporated in the composite scanner 6 is an eddy current sensor,generally indicated at 22, comprised of differential windings 24, 26housed in a body 28 of insulating material such as nylon, carried by andsecured in the sheath 16. Leads 30 running through the body 10 and cable8 transmit the signals generated by the eddy current sensor to thesignal receiving instrumentation. In traversing a tube, the cusps 14, asshown in FIG. 1, bear against the interior wall causing each springfinger 12 to bend from the predetermined null or normal position inaccordance with variations in tube profile.

In FIG. 5, a circuit configuration is shown whereby, in traversing atube, the deflection of each spring finger 12 from a null or neutralposition may be determined, or the algebraic sum of the deflections ofany two spring fingers from a null or neutral position may bedetermined. As ordinarily it is the algebraic sum of the deflections ofdiametrically opposite spring fingers which is of interest, there isshown in FIG. 5, the eight spring fingers incorporated in four bridgecircuits identified as circuits W,X, Y, Z and energized from a source ofpotential 44, wherein the diametrically opposite strain gauges 18 arearranged in push-pull relationship. The potential difference, or bridgeunbalance, as it may be termed, appearing across leads 45-46 of any oneof the bridge circuits is therefore a measure of the algebraic sum ofthe deflections of the diametrically opposite spring fingers 12. It isapparent that the algebraic sum of the deflections of any two springfingers, as for example, adjacent spring fingers, can be determined bythe potential difference between any two leads 45, two leads 46, or anycombination of leads 45, 46.

Also incorporated in the circuit configuration is a voltage divider 48generating a predetermined potential appearing in lead 50. Accordingly,changes in the difference in potentials between lead 50 and any one ofthe leads 45 or 46 of bridge circuits W, X, Y, Z will be a measure ofthe displacement of the spring finger from a null or neutral position intraversing a tube. From the displacements of all of the spring fingersthe shape of the tube at any one or all points in a traverse can bedetermined.

The potential in lead 50 and the potentials in leads 45, 46 for each ofthe bridge circuits W, X, Y, Z collectively indentified as leads 21 inFIG. 5 are transmitted to suitable read-out devices such as shown inFIG. 6. Signals corresponding to bridge unbalances are generated indevice 52, the number of such signal generators depending upon theparticular type of tube scan. Thus, four such signal generators would berequired to simultaneously read out the algebraic sum of the deflectionsof eight diametrically opposite spring fingers, whereas eight suchgenerators would be required to simultaneously read out the deflectionsof each spring finger from a null or neutral position. The signalsgenerated in 52 can be transmitted to a variety of read-out devices asdictated by a particular application. As examples of such devices thereis shown in FIG. 6 a tape recorder 36 and a strip chart recorder 38responsive to the signals generated in the device 52. From the read-outdevices such significant data as the inside tube radial dimensions ateight points at any one or all points in the tube traverse can beplotted or digitized.

The electrical connections from the eddy current sensor 22 are carriedthrough leads 30 to an eddy current tester 32, such as aZetec/Automation EM 3300 Eddy Current Tester. The signature appearing onthe scope of the tester, as the scanner 6 traverses a tube, will varydepending upon the character of the tube, a minor horizontal deflectionis obtained representative of sensor wobble, whereas tube abnormalitiessuch as a crack shown at 34 in FIG. 1 will generate on the scope apattern from which the type of flaw can be determined. In traversing thetube adjacent a support plate, a fat two lobed signature of the supportplate is generated as discussed in U.S. Pat. No. 4,194,149. The abruptchange in either the horizontal or vertical component of the eddycurrent signature available at terminals 39, 40 at the nearest supportplate can therefore be taken as a bench mark, from which, knowing thespeed of the scanner in traversing a tube, the exact location of a tubedefect can be determined. To provide proper correlation, as shown, thesignals available at terminals 39, 40 are transmitted to the taperecorder 36 and strip chart recorder 38 simultaneously with the readingsfrom the strain gauges.

In FIG. 7 there is shown a fragment of typical chart traces of onecomponent, either horizontal or vertical, of an eddy current sensor andthe signal output of one strain gauge bridge or a single spring fingergenerated by the composite scanner 6 in traversing a tube. As noted, theeddy current signal abruptly changes at a support plate and provides abench mark by which, knowing the ratio between scanner and chart speeds,the location of a dent, such as shown at 42 and 43 in FIG. 1, relativeto the support plate 4 can be accurately determined. Other tubeabnormalities such as variations in diameter out-of-roundness and thelike encountered during a traverse can be accurately located. Thephysical dimensions and contour of the abnormality can be determined byan analysis of the signal outputs as recorded on the tape recorder 36 orthe chart recorder 38.

With the construction shown in FIG. 2, because of the axial displacementof the eddy current sensor 22 from the cusps 14, there must be made acorrection proportional to this axial displacement, to the distance asindicated or recorded by the read-out equipment shown in FIG. 6. In FIG.2A there is shown a modified construction of the scanner 6 whereby thenecessity for such a correction is eliminated. As shown therein, thereis a body 10A made of a non-conducting, non-metalic spring materialprovided with contilevered spring fingers, which may be formed integralwith the body or securely anchored thereto by any suitable means. Nearthe free end a section of each finger 12A is bent to form the outwardlyextending crook 13, necked down to form the cusp 14. Preferably, thefingers 12A are enclosed in a protective sheath 16A, also made of anon-conducting, non-metalic insulating material secured to the body 10Aand provided with openings 17 through which the crooks 13 project.

An eddy current sensor 22A is housed in a disc 27 secured to a hollowtube 29 through which leads 30 run. The tube 29 is supported by andaxially movable in a body or nose piece 28A and a spider 31, each madeof an insulating material. The eddy current sensor 22A is located in aplane normal to the longitudinal axis of the sensor and which passesthrough the cusps 14 thereby eliminating the correction required indetermining the axial locations of tube abnormalities. Due to electricaldistortions, it may be necessary to have the sensor 22A slightlydisplaced from the plane of the cusps 14 so that the abrupt change ineither the horizontal or vertical component of the eddy current willoccur coincident with the cusps 14 taking the same position relative toa support plate. To facilitate such minor adjustments, the nose piece28A may be provided with a boss 54 and set screw 56, by which, after thedesired axial location of the eddy current sensor 22A has beendetermined, the tube 29 can be clamped and firmly held in position.

I claim:
 1. In an apparatus for inspecting a heat exchanger tube, acomposite scanner adapted to be drawn through the tube having acylindrical body portion, a flexure spring having one end anchored tosaid body portion adjacent its outer circumference and a free endcontacting the interior wall of the tube as the scanner is drawntherethrough, a strain gauge operatively connected to said flexurespring having a winding responsive to the flexing of said flexure springgenerating a first signal corresponding to the response of said windingto the flexing of said flexure spring, and an eddy current sensor havinga winding incorporated in said scanner electromagnetically coupled withsaid tube generating a second signal corresponding to changes in saidelectromagnetic coupling as said scanner is drawn through said tube. 2.Apparatus as set forth in claim 1 further including a cylindrical shroudsecured to said body portion around its outer circumference surroundingsaid flexure spring and having an aperature through which the free endof said flexure spring extends, and a closure for the open end of saidshroud remote from said body portion having a cylindrical sectionfabricated of an insulating material in the circumference of which thewinding of said eddy current sensor is disposed.
 3. Apparatus as setforth in claim 1 further including a disc of insulating material in thecircumference of which the winding of said eddy current sensor isdisposed, and means supporting said disc concentric with the axis ofsaid body portion and in a plane perpendicular to the axis of said bodyportion.
 4. Apparatus as set forth in claim 3 wherein said meanssupporting said disc comprises a rod on which said disc is mountedsupported by and axially movable relative to said body portion and thefree end of said flexure spring.
 5. Apparatus as set forth in claim 4wherein said disc is disposed in a plane passing through the free end ofsaid flexure spring.
 6. Apparatus as set forth in claim 1 wherein saidtube is provided with a contiguous support plate maintaining the tube inpredetermined position and effecting a unique change in theelectromagnetic coupling between the winding of said eddy current sensorand said tube as the scanner traverses the tube adjacent to said supportplate to thereby generate a unique change in said second signal andestablish a bench mark identifying a point in the travel of said scannerthrough said tube.
 7. Apparatus as set forth in claim 6 wherein saidtube is provided with a plurality of spaced apart support platesmaintaining the tube in predetermined position and each effecting aunique change in the magnetic coupling between the winding of said eddycurrent sensor and said tube as the scanner traverses the tube adjacentto each of said support plates to thereby establish a plurality of benchmarks equal in number to the number of support plates traversed by thescanner.